The present invention relates to a solid oxide fuel cell with external manifolds.
Conventional solid oxide fuel cell stacks are formed from stacked interconnect plates, also known as bipolar plates, fuel cells comprising membranes and electrodes, and seals. The interconnects and the fuel cells are typically planar and define air and fuel intake and exhaust openings. When stacked vertically, the openings define the intake and exhaust manifolds. The interconnect plates have internal passages on either side of a central barrier which directs air or fuel from its intake manifold, across the fuel cell electrode and into the exhaust manifold. Typically, the fuel cell is square and the fuel gas flows in a direction perpendicular to the direction of air flow across the cell.
Up to five gasket seals are required on either side of an interconnect: one for each manifold and one to surround the electrode surface of the fuel cell. The seals pose a significant hurdle for efficient fuel cell operation as they must provide adequate gas seals while being somewhat compressible, flexible and tolerant of heat cycling within the fuel cell stack. This combination of interconnects and seals necessitated by the internal manifolds of prior art fuel cells creates numerous difficulties which require expensive and complex solutions.
External manifolds are known but also suffer from disadvantages. A significant problem is that the sealing surfaces on the sides of the stack are irregular due to inherent variances in cell sizes. This irregular surface is difficult to seal against and any seal that is developed is often compromised during any thermocycles due to mismatches in thermal expansion.
A significant disadvantage of many external manifold designs is that they require compression along all 3 of the X, Y and Z axes to seal the stack—compressive load in the vertical Z direction to seal the cells to the flow separators and for electrode contact, as well as compression in the X and Y directions to seal the manifolds to the stack. X-Y compression techniques include band clamps, and bolts with low thermal expansion but these techniques may suffer from material creep over time and eventually fail to hold the manifolds tightly to the stack.
Therefore, there is a need in the art for a fuel cell stack with external manifolds which may mitigate the difficulties of the prior art.